Caption: Polyarteritis nodosa in a kidney. Light micrograph of a section through a kidney, showing a blood vessel affected by polyarteritis nodosa (PAN). PAN is an uncommon condition that causes vasculitis (blood vessel swelling) of medium & small-sized arteries, which become swollen and damaged from attack by rogue immune cells. The cause of PAN is not well understood but may be associated with hepatitis B virus.